Detailed documentation is actually in the Jupyter container!
Read XXX.c3.ai/jupyter/notebooks/tutorials/TutorialIntroJupyterNotebook.ipynb

TLDR:
If you want to explore a new runtime/kernel, create a yaml file in requirements_files folder. If you want to add (productionize) a new runtime/kernel, create ActionRuntime seed data in the codebase and re-deploy (provision) the code.

Either way, you just need to refresh the jupyter container page again, then you will see your kernel/runtime show up in the upper right corner dropdown. Since it is a lazy installation, when you click it the first time, it will install all the packages required.

Note that all content in the tree view is shared within the tenant/tag. However, the installed kernel is specific to one’s container